The goal of the Animal Resources Core will continue to be to generate and maintain transgenic mouse lines for use in subprojects. Following the successful generation of a number of mouse lines during the previous granting period, the efforts of the Animal Resources Core will be shifting towards maintenance and breeding of existing transgenic mouse lines. The Core will continue to provide microinjection services for generating additional lines of mice to fulfill the goals of the research projects. This service will be primarily directed towards injections of genetically manipulated embryonic stem cell lines into blastocysts, but will also offer pro-nuclear injections of DNA when needed. Based on the predictions of the principal investigators, the Core expects to provide microinjection services for twelve to fifteen cell lines each year. The Core will be directing more of its efforts into freezing embryos from lines of transgenic mice in order to reduce animal care costs and to facilitate transfer of mice between institutions. During periods between microinjections, the breeding stocks and mice used for generating pseudopregnant females will be used for testing conditions for producing embryos for freezing and for testing frozen stocks of embryos. A CryoLogic CL 863 programmable controlled rate freezing unit has been purchased for use by Core C personnel and initial experiments to test freezing protocols have been successful. The primary function of the Core will be to maintain breeding stocks and produce study animals for the various research projects. The Core oversees breeding plans, performs DNA typing, maintains breeding records and coordinates with investigators for the production of animals in the required numbers and at the appropriate ages for experiments. The Core also maintains a line for FVB/N mice carrying a pSV40tkneo construct that is used to produce neomycin resistant embryonic fibroblasts for culture and selection of embryonic stem cells. One new function of the Core will be to maintain a colony of TS65Dn for use by investigators within the program at the Johns Hopkins School of Medicine. The TS65Dn colony was obtained from the laboratory of Dr. Muriel Davisson and is maintained according to the same breeding protocols. Genetic typing of the mice will be performed FISH analysis of blood from trails using a rapid and convenient protocol developed by Core D: Cytogenetics. All animals will be maintained in micro-isolator cages in a pathogen-fee facility at the Johns Hopkins School of Medicine. The Core has the ability to import lines of mice from external facilities as needed by performing embryo transfers with sentinel follow-up.